Simplified avidity assay of rubella IgG antibody with urea was evaluated to distinguish primary rubella from reinfection. In this method urea washing was done once for 10 minutes. The avidity index (AI) was calculated as the optical density percentage for the urea-washed well when compared to that of the non-treated well. We examined 292 sera from 50 patients with primary infection collected 6 to 2,259 days after the rash appeared, 29 sera from 11 patients with rubella reinfection and 69 sera from 68 pregnant women without fetal infection and having a high hemagglutination inhibition (HI) antibody. In primary infection AI increased gradually from 0%, and reached a plateau of about 60% four months after the rash appeared, whereas the mean AIs of patients after reinfection and with high HI antibody were as high as 87.1% and 89.9%, respectively. These results indicate that the simplified avidity assay in rubella IgG antibody is also valuable in diagnosing recent primary rubella in pregnant women with a high HI antibody.